Found by divers in a Mexican cave, the 12,000-year-old bones of a teenage girl are the first intact remains of a Paleolithic American.

One day about 12,000 years ago on Mexico's Yucatan Peninsula, a small 15-year-old girl with a narrow face, wide-set eyes, a broad nose, and a high forehead, entered a cave. No one knows who she was or what had brought her there, but in those dry days of looming glaciers and low sea levels, it's poss...